Make render preview faster
You can use ctrl-b to draw a rendering box, so that you preview doesn't have to draw the whole view.



You can enable Simplify in properties/scene, which will allow you to create separate settings for subdivision for preview, or to create a hard limit on light bounces so that they'll (as in this pic) just skip to AO (white world) after 2 bounces instead of doing more.

And of course, you can do any of the things that you do to keep renders fast.  I avoid shaders I know run slow (volume scatter, glass, SSS) mostly because of their effect on my preview.